Location of faults in electrical transmission systems



March 7, 1950 R. A. KEMPF 7 2,499,759

LOCATION OF FAULTS IN ELECTRICAL TRANSMISSION SYSTEMS Filed July 31,1947 FIG. /A

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IN V E N 70/? By R. A KEMP/'- ONE 510: OPPOSITE 5/05 sr/uoau? or FAULTor nun FAULT ATTOR EV Patented Mar. 7, 1950 AtQJSQ LOCATION OF FAULTS INELECTRICAL TRANSMISSION SYSTEMS Raymond A. Kempf, Baltimore, Md,assignor to Bell. Telephone Laboratories, Incorporated, New York, N. Y.,a corporation of New York Application July 31, 1947, Serial No. 765,0876 Claims. (Cl. 175-183) This relates in general to the location offaults in an electrical transmission system. More particularly, itrelates to the location of high-voltage faults in electricaltransmission lines.

In certain types of coaxial cable systems, breakdown at high voltage isfrequently caused by the presence of mechanical defects or metallicinclusions which may not be located by the usual prior art methods.Moreover, in accordance with certain other prior art systems, the faultypoint is located only in terms of length ratios whereby the region, butnot the exact point of fault, is known. If the length is not preciselyknown, the cable system must be opened over a considerable distance forvisual inspection and listening tests, which procedure is timeconsuming, particularly from the standpoint that manual rebuilding ofthe inspected. section frequently causes mechanical and electricaldegradation of the cable system.

It is therefore the primary object of this invention to providetechniques and apparatus for locating the precise physical points ofhighvoltage faults in electrical transmission systems.

A particular feature of the present invention is its adaptability forthe large-scale factory inspection of certain types of cable forhigh-voltage faults. The invention is particularly useful for thelocation of high-voltage faults in disk-insulated longitudinal seamcoaxial cable, whether in single or twisted multiple strands, and alsoin other types of cable circuits such as lightning protected cablehaving a sheath-to-corrugatedcopper path.

In accordance with the present invention, the test cable is fed from oneto another of a pair of reels, condensers being slidably connectedbetween the central conductor and sheath through slip rings at each endof the test interval. A high enough direct-current voltage is applied toone end of the circuit toproduce an electromagnetic disturbance whichmay take the form of a flash-over at the fault. The intermittentdischarges of one or both condensers through a flash-over point at thefault produce 1. R. drops along the sheath which can be measured andphase-compared in an indicator, such as a cathode-ray oscilloscope,which is electrically coupled to the cable through a rolling or slidingpick-up device. The exact location of a fault can then be effected by adetermination of the point at which phase reversal occurs on theindicator as a current detecting means or, what is more commonly knownin the engineering vernacular as a pick-up device is moved. over thetest inter val. Several alternative forms of pickup and indicatingcircuits are disclosed.

The invention will be better understood and other objects and featuresthereof will be apparent from a study of the drawings and the detaileddescription set forth hereinafter.

In the drawings:

Fig. 1A shows a high-voltage fault locating system in accordance withthe present invention mounted for examination of cable passing betweentwo reels;

Figs. 1 (B-E) show modified forms and positions of the pick-up device I9of Fig. 1A;

Fig. 1F shows a further modification of the system of Fig. 1A in which agalvanometer is substituted for the cathode-ray tube indicator 25;

Fig. 2 is a schematic diagram of the equivalent electrical circuitindicating the principle of operation of the system of Fig. 1A; and

Figs. 3 (A), 3 (B) and 3 (C) show various types of figures which appearas the screen of the cathode-ray tube indicator 25 during operation ofthe system of Fig. 1A.

Referring to Fig. 1A of the drawings, one of the preferred embodimentsof the invention will now be described in detail.

The test cable I may be a longitudinal seam coaxial cable of the typeand size conventionally used for telephone communication, comprising acylindrical outer conductor 2 and an inner axial conductor 3 which aremaintained at a uniform separation by means of thin disks of dielectricmaterial such as hard rubber or polyethylene disposed at regularintervals along the interior of the cable. ihe dielectric medium is thusalmost wholly gaseous.

Assume the presence in the test cable 1 of one or more mechanicaldefects or metallic inclusions 4 which tends to increase the potentialgradient between the outer conductor 2 and the inner conductor 3 at thatparticular point, making it more susceptible to high-voltage breakdown.

The cable 5 is mounted on a pair of reels 5 and 6 which are spaced apartby a convenient distance X, a few yards long. The reels 5 and 6 are sodisposed as to be rotatable in a clockwise direction, whereby the cableI may be progressively fed at any desired rate from the reel 5 to thereel 6, moving from left to right across the space interval X.

One terminal of the sheath 2 of the test cable section i is connected tothe slip ring 5a of the reel 5, and the other terminal of the sheath 2is connected to the slip ring 6a of the reel 6. Likewise, the terminalsof the inner cable con- 3 ductor 3 are respectively connected to theslip rings 52) and 6b of the reels 5 and 6.

A source of direct current potential H which may comprise, for example,a kenatron tube or other high voltage source, is adapted to be connectedbetween the inner conductor 3 and the sheath 2 of the cable I at theright-hand reel 6. The connecting circuit includes the sliding contacts1 and 8 riding on the slip rings 6b and 6a thereby respectively makingcontact with the inner conducting cable 3 and the cable sheath 2; thecondenser ii the regulating resistance 12, and the interconnectingswitches l3 and I4.

Switches 13 and I4 are so adapted that the source H in parallel with thecondenser i is connected between the inner and outer cable conductors 3and 2 when switch 13 is positioned on contact He and the switch 14 ispositioned on the contact Mb.

Alternatively, the source H is adapted to be independently connected tocharge up the condenser IO when the switch I4 is in position I41: andthe switch [3 is in position I37). In accordance with a thirdarrangement the switch 13 may be positioned so that the condenser I 0 isdisconnected from the circuit and the source H is connected directlybetween the sheath 2 and the inner conductor 3 of the cable I throughthe contact I41) of the switch I4.

At the left-hand reel 5, a condenser I1 is provided for connectionbetween contacts l and I6, and hence between the sheath 2 and the innerconductor 3 of the cable I. The switch 18 provides for alternativelyconnecting condenser I! in and out of the circuit.

The cable I is threaded through a pick-up device |9 which is coupled toa detecting device 20. The pick-up device I9 comprises an inductivestrap 23 mounted on the rolling contactors 2| and 22 which are adaptedto move over the surface of the cable sheath and make electrical contacttherewith.

The detecting device may comprise a cathode-ray oscilloscope having ahigh-persista-nce screen and provided with vertical deflecting plates 26and horizontal deflecting plates 21. The vertical deflecting plates 26are connected through the delay circuit 30 to the coil 24 which isinductively coupled to the contacting strap 23.

The horizontal deflecting plates 21 are connected across the output ofthe sweep generator 28, which is a conventional type of saw-toothvoltage generator well known in the art. The sweep generator 28 istriggered to synchronize its operation with recurrence offault-generated vibrations by connection to the cable output through acircuit which includes the condenser 29 in series with the inductivecoupling coil 24.

Assume the system of Fig. 1 is to be operated for the purpose ofexamining a length of cable I for high voltage defects of the typedescribed.

The cable I is mounted on the reels 5 and 6 in the manner described sothat upon rotation of the reels 5 and B in a clockwise direction thecable is progressively threaded through the rolling contactors 2| and22. In the circuit associated with the right-hand reel 5 the switch I4is positioned on contact Mb; and the switch i3 is positioned on contact13a, thereby connecting the potential source II and the condenser if! inparallel between the central conductor 3 and the outer conducting sheath2. In the circuit associated with the left-hand reel 6, the switch 18 isclosed, connecting the condenser l1 between the central conductor 3 andthe sheath 2.

Assume the presence in the test interval X of a mechanical defect ormetallic inclusion 4 which tends to increase the potential gradientbetween the outer and inner conductors at a particular point, making itmore susceptible to high voltage breakdown. If the potential of thesource I! is adjusted to a sufiiciently high value, a voltage breakdownoccurs at the fault 4 which takes the form of a series of periodic arcdischarges between the inner and outer conductors, whose rate isdependent on the constants of the R. C. circuit comprising the cableconductors 2 and 3, the condensers l0 and IT, and the rheostat l2, whichmay be adjusted to secure the desired timing. The aforesaid dischargesinitiate trains of waves which travel along the cable conductors in bothdirections from the fault 4.

When discharge takes place, the condition shown in the equivalentcircuit of Fig. 2 will obtain. Condensers [Band I! periodicallydischarge through the flash-over or breakdown at 4 each time it occurs,and each develops a measurable IR drop along the outer conductor of theunit, where I is the current in the coaxial conductors and R is thesurface transfer impedance of the outer conductor. Inasmuch as thecondensers discharge from opposite ends of the unit, the individual IRdrops developed are in series opposition, with the fault forming thejunction of common polarity.

It has been observed experimentally that it is important to keep theimpedance of the exterior circuit between the ends of the coaxial cableI relatively high as compared to the impedance of the strap 23. Thisexternal circuit is shown on the equivalent circuit of Fig. 2 by thedashed line. The current Is in the external circuit is set up wheneverthere exists an inequality in voltage drops along the outer conductors,as a result of the position of the point of breakdown relative to thereels. The direction of flow of I3 has been found to depend on theposition of the fault with respect to the reels; that is, the directionreverses as the fault is re-reeled from one reel to the other. When thefault is between the reels the current I3 flows in proportion to thedifference in the two voltages along the outer surface of the outerconductor produced by the currents I1 and I2 acting through the surfacetransfer impedance of the outer conductor. Thus, I3 reaches a null whenthe fault is exactly centered between the reels. It has been foundexperimentally that a pick-up coil or loop placed in the vicinity of thecable appears to pick up a voltage which is induced by the current I3.Obviously, the induced voltage does not change polarity when the coil ismoved past the fault, because I3 flows around the circuit including thetertiary impedance indicated by the dashed line on the drawing. It isthis fact that explains the need for the low-impedance strap. The strap23 provides an impedance path much lower than that through the tertiarycircuit, and thus the current through the strap will reverse when thefault is passed. The pick-up coil is coupled to the strap as indicatedon the drawing.

By rotation of the reels 5 and 6 the test cable I is fed from left toright across the interval X so that it progressively threads through therolling contactors 2| and 22 of the inductive strap 23. Assuming thepresence of a fault 4 in the test interval X, the screen of theoscilloscope 25 will show a deflection which changes in polarity as thestrap 23 passes the fault, such as shown in- Figs. 3Aand-3B.

As indicated in Fig. 3C, no: deflection will be obtained when the strapis centered over the fault.-

This method thus facilitates precise physical: location of the faultorpoint of flash-over at any convenient speed of re-reeling.

In accordance with an alternativeform of operation either one or theother of the condensers ill or ll may be disconnected from the circuitby open-circuiting their respective switches l3 and I8. In this case,the oscilloscope deflection will be obtained only when the pick-updevice 19 is located between the operating condenser and the fault 4.

Short circuits may be located by eliminating the condenser at the farend, and connecting switches l3 and 14 so that the condenser Ill may bealternatively charged and discharged through the cable connection. Theresultant discharge may be detected by a deflection appearing on thescreen of the oscilloscope 25" when the pick-up device is locatedbetween the condenser l and the fault 4.

Although it is conceivable that a system in accordance with the presentinvention might be operated without either of the externally connectedcondensers ill and H by placing dependence on the charge collected alongthe distributed capacitance of the cable I for generation of detectablecurrents I1 and I2, these currents would be small in magnitude and highin frequency with consequent difficulties.

Other devices for picking up the IR drop along the outer conductor orthe field set up by the discharge current flowing in the conductors maybe substituted for the pick-up u'nit i9 of Fig. 1A, such as shown inFigs. lB-lF. All of these devices depend upon either the current fiowaccompanying the breakdown at the fault to set up a magnetic field whichmay be detected, or upon the potential difference between adjacentsections of the outer conductor to set up detectable current fiow in asecondary or tertiary circult which is attached to the test coaxialcable unit.

The detectable field has been shown to be dependent upon the relativeconcentricity of the conductors of the coaxials in addition to theshielding properties of the outer conductor. In the case ofeccentricity, the resulting external field may over-shadow that normallypresent in the absence of eccentricity. Since the degree and directionof eccentricity is likely to occur at random along a given coaxial, thesignal detected by the pick-up device may vary over wide limits leadingto inconclusive or misleading fault location. Use of a strap such as 23in Fig. 1A eliminates this confusion.

The pick-up device shown in Fig. 1B, which may be substituted for thepick-up unit [9 of Fig. 1A, comprises a conducting strap 32 havingcontacting probes 32a and 321) which are adapted to be moved along thecable sheath 2. The fault generated signal is picked up inductivelythrough a solenoid 3i having its axis substantially at right angles tothe longitudinal axis of the cable I. The solenoid 3| may be connectedto any suitable polarity-sensitive indicator such as the cath0de-raytube indicator 2!] of Fig. 1A.

Another alternative form of the pick-up device H! is shown in Fig. whichutilizes a toroid 35 wound on a high permeability core which is threadedthrough a conducting strap 34 having rolling contactors 34a and 34bwhich make electrical contact with the cable sheath 2. Instead 6. of thetoroid; 3.6: a single-loop of wire 3.8 asshown in: Fig. 1CD): may beinductively coupled to; a. tertiary circuit including the conductingstrap 37 and the cable sheath 2.

Another alternative form of the pick-up device #9 is shown in Fig- 1E,in which a capacitative coupling with the cable sheath 2 is substitutedfor the direct contact coupling between the sheath and conducting strap,such as shown in Figs. lA-l'D. The contactors 4t and 41 comprise shapedmetal shoes lined with dielectric material which are adapted to rideover the cable sheath 2- without making direct electrical contact:therewith. The contactors All and 4! are connected to an indicatingdevice such as the cathode-ray tube indicator 20 described withreference to- Fig. 1A above.

Fig. 1F shows a special type of coupling in which the coaxial under testis threaded through a solenoid. The voltage at terminals of the solenoid45 willbe zero unless the coaxial cable I is wrapped with helicallyapplied steel tapes 42, as in the case of the standard telephonecoaxial, or other conductor applied in direct contact with the copperouter sheath 2. The helical steel tapes or other conductors have currentset up in them due to potential differences longitudinally along thecoaxial sheath 2. Since the tapes or conductors form an elongatedsolenoid, they will induce current in. another solenoid, such as 45,which has the same or a parallel axis. Under such conditions, the us ofthe steel or other helical conductors to induce voltage in the pickupcoil is in reality a special case of the use of the strap 23' disclosedin Fig. 1A.

The terminals of the coil 45 are coupled to a polarity-sensitiveindicating device, which may take. the form of a cathode-ray tubeindicator such as Zll described hereinbefore with reference tov Fig.1A,. or alternatively, a conventional galvanometer such as 46.

It is apparent that the galvanometer 46 could be substituted for thecathode-ray tube indicator 20- in combination with any of thealternative pick-up devices shown in Figs. 1A-1E. Moreover, it will beapparent to those skilled in the art that within the scope of theinvention other equivalent elements and combinations of elements can beused in addition to those disclosed; and the test system is applicablefor the testing of other cable units than the type described, such as,for example, stranded cables having two or more coaxials, orlightning-protected cable having corrugated copper sheathing, orflexible coaxial cable having a braided outer conductor. In fact, thesystem of the present invention is adapted for the location of any coreto sheath failure in a cable having a multiplicity of conductors suchas, for example, a paper insulated voice frequency or carrier frequencytelephone cable.

What is claimed is:

1. The method of locating a high voltage conductor-to-sheath fault in aconductively sheathed cable section which comprises storing electricalcharge between said conductorand said sheath of sufficient magnitude tocause repeated voltage breakdown at said fault whereby said storedcharge is repeatedly discharged through a circuit including said faultand a portion of said cable section adjacent thereto, and detectingvariations in said discharge current in the sheath at difierent pointsalong said cable section for determining the physical location of saidfault.

-2. The method of locating a high-voltage conductor-to-sheath fault in aconductively sheathed cable section which comprises repeatedly storingelectrical charges between conductor and sheath at both ends of thesection of sufiicient magnitude to cause repeated voltage breakdown atthe fault whereby said stored charges are repeatedly discharged throughthe fault and the two intervening portions of cable, and detecting therelative phase of the discharge current in the sheath at difierentpoints along said cable section to determine the point at which saidrelative phase reverses.

3. A system for locating faults in a transmission line comprising aplurality of conductors which comprises in combination means forproducing periodic capacitance discharges between certain of saidconductors at a fault in said line whereby current surges are caused toflow from said fault through a circuit including a section of at leastone of said conductors adjacent said fault, a sheath current detectingcircuit positioned to move along said conductor in a directionsubstantially parallel to the direction of current flow therein, and apolarity-sensitive current indicating circuit electrically coupled tosaid detecting circuit to indicate variations in the magnitude and phaseof said current at diiferent points on said conductor.

4. A system for locating faults in a transmission line comprising aplurality of conductors which comprises in combination means forproducing periodic capacitance discharges between certain of saidconductors at a fault in said line whereby current surges are caused tofiow in both directions from said fault along at least one of saidconductors, a sheath current detecting circuit positioned to move alongsaid conductor in a direction substantially parallel to the direction ofcurrent flow therein, and a polarity-sensitive current indicatingcircuit electrically coupled to said detecting circuit to indicatevariations in the magnitude and phase of said current at differentpoints on said conductor.

5. A system for testing conductively sheathed cable which comprises incombination means to progressively feed lengths of said test cableacross a preselected test interval, at least one condenser slidablymounted in circuit relation to said cable at one end of said testinterval, a source of direct current potential connected in circuitrelation to charge said condenser to a sufliciently high potential tocause arcing at a fault in said cable passing through said interval, asheath current detecting circuit slidably mounted to move along saidcable within said interval, and a polarity-sensitive indicating circuitcoupled to said detecting circuit.

6. A system for testing conductively sheathed cable which comprises incombination means to progressively feed lengths of said test cableacross a preselected test interval, a pair of condensers each of whichis slidably mounted in circuit relation to said cable at a. differentend of said interval, a source of direct current potential connected incircuit relation to charge said condensers to a sufiiciently highpotential to cause arcing at a fault in said cable passing through saidinterval, electrical pick-up means slidably mounted to move along saidcable within said interval, and polarity-sensitive electrical indicatingmeans electrically coupled to said pick-up means.

RAYMOND A. KEMPF.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,745,419 Henneberger Feb. 4,1930 1,886,682 Hubbard Nov. 8, 1932 2,176,757 Borden Oct. 17, 19392,199,846 Borden May 7, 1940 2,321,424 Rohats June 8, 1943 2,460,688Gambrill et a1. Feb. 1, 1949 FOREIGN PATENTS Number Country Date 306,679Germany July 9, 1918

